Natural gas is often produced, either in association with crude oil or on its own, in a remote location where it is not economic to transport the gas to use points with pipelines. One method of recovering this gas is to convert it to paraffinic hydrocarbons or their oxygenated derivatives by catalytic hydrogenation. The term "catalytic hydrogenation" is used in this Application to mean contacting the gas with a catalyst whereby carbon monoxide in the gas is hydrogenated by the hydrogen content of the gas to produce paraffinic hydrocarbons or oxygenated derivatives, especially methanol and dimethylether but also aldehydes and ketones. The term "Fischer-Tropsch process" means such catalytic hydrogenation to prepare paraffinic hydrocarbons. Other uses of synthesis gas include the preparation of ammonia. These gas conversion processes are exothermic and conventionally the heat of reaction is used to generate steam.
The conversion of natural gas (or other hydrocarbon feedstock) to synthesis gas is accomplished using one of several standard methods available such as steam reforming, autothermal reforming, combined steam and autothermal reforming, combined reforming with pre-reforming partial oxidation and gas heated reforming etc. The most favourable systems to minimise overall equipment size and cost of production use processes in which oxygen is separated from air and used in, for example, an autothermal, combined reformer or partial oxidation system. The use of oxygen allows the production of synthesis gas having the high CO:H.sub.2 ratio (about 1:1.6 to about 1:2.5) required for a catalytic hydrogenation process without excess hydrogen production.
U.K. Patent publication No. 2237287 A discloses a methanol synthesis in which the feedstock is provided by reformed natural gas. The reforming is conducted with oxygen-enriched air and part of the reformed gas is subjected to a carbon monoxide shift reaction and the hydrogen content of the resultant gas stream mixed with the remainder of the reformed gas to provide a hydrogen-enriched feedstock for the methanol synthesis.
European Patent publication No. 0634562 A discloses an integrated air separation-gas turbine power generation process in which synthesis gas is combusted with saturated, compressed gas turbine feed air to provide a combustion gas which is work expanded in the gas turbine to drive gas turbine feed air compressor(s) and provide power. The synthesis gas is prepared by reaction of a carbonaceous fuel with compressed air separation unit ("ASU") oxygen product. At least part of the ASU feed air is provided from a gas turbine compressor. The water used to saturate the compressed gas turbine feed air prior to combustion is heated using the heat of compression of the ASU oxygen product prior to synthesis gas production.
U.S. Pat. No. 4,888,131 (Goetsch et al) and U.S. Pat. No. 5,160,456 (Lahn et al) both describe the partial oxidation of natural gas with oxygen to produce synthesis gas and the conversion of that gas into liquid products by a Fischer-Tropsch process ("remote gas process") but do not describe the ASU used. Further information on the remote gas process is provided in a paper by Ansell et al ("Liquid/fuels from Natural Gas--An Update of the Exxon Process") presented at the Council on Alternate Fuels, Apr. 26-28, 1994 but again there are no details of the ASU.
A paper by Tijm et al ("Shell Middle Distillate Synthesis The Process, The Products, The Plant") presented at the Council on Alternate Fuels, Apr. 26-29, 1994 gives a summary of the remote gas process developed by Shell, but does not provide any details of the ASU.
A paper by Choi et al ("Design/Economics of a natural gas based Fischer-Tropsch plant") presented at the AIChemE 1997 Spring National Meeting Mar. 9-13, 1997 gives further economic and design information of a remote gas process but no information on the ASU.
International Patent Application No. WO 97/12118 (PCT/NO96/00227) has a detailed process description of, and provides heat and material balances for, a remote gas process but again fails to provide ASU details.
U.S. Pat. No. 5,635,541 (Smith et al) describes a remote gas process in which part of the steam generated by the gas conversion reaction is used to drive the gas compression requirements of an elevated pressure ASU.
Conventionally, the air feed for an ASU is compressed in a generally isothermal manner using a multistage compressor with intercoolers. Little, if any, useful energy can be recovered from the intercoolers. However, U.S. Pat. No. 4,461,154 (Allam) teaches that, at a sufficiently high compression ratio (at least 2.5:1), generally adiabatic compression of a gas produces a temperature sufficiently high to provide high grade energy which can be used directly or indirectly to assist in driving the compressor. The energy produced compensates for the additional power required for the adiabatic compression.
In particular, U.S. Pat. No. 4,461,154 describes an air compression system having a steam turbine drive in which the air is compressed adiabatically; the compressed air leaving the compressor is passed through an aftercooler where it heats boiler feedwater; the heated boiler feedwater is vaporized in the boiler producing superheated steam; and the steam used to drive the air compressor. The preheating of the boiler feedwater allows the excess heat in the boiler flue gas to be used for preheating air fed to the boiler for fuel combustion. The arrangement allows a substantial reduction in the fuel consumption for steam production. Further, the weight, volume, cost, footprint, and height of an adiabatic compressor are all considerably smaller than for a multistage compressor with intercoolers as used in conventional ASUs.
It is an object of this invention to integrate an ASU with synthesis gas production and conversion in a cost effective manner suitable for remote gas processing. It is a further object of the invention to integrate air separation and natural gas utilization for ease of installation on an offshore gas or gas/oil production platform or floating production storage and offloading system (FPSO). Other objects are to minimise the volume and weight of the ASU; to minimise the cost of the total facility power system by optimising the backup power supply required for start-up and operation of the ASU; and to more efficiently utilise the excess energy (usually in the form of steam) available from the gas conversion. Some or all of these objectives are achieved in varying degrees by processes of the present invention. Although the invention has been developed to meet these objectives, it is not limited to application in remote locations or to the use of natural gas but is of general application where appropriate conditions and requirements exist.